In the field of forensic science, investigations of crimes involving firearms use ballistic comparison tests to determine if a bullet or a spent cartridge case found on the crime scene has been fired by a firearm in question. Ballistic comparison tests rely on the striations and/or impressions that are created on the surface of a piece of evidence when a firearm is fired. These striations and/or impressions have enough unique features to represent a signature of the firearm. Therefore by comparing the striations or impressed characteristics of two bullets or two cartridge cases, it is possible to conclude if they have been fired by the same firearm. Similarly, by comparing the striations and/or impressions on two objects showing tool marks resulting from cutting, prying, hammering or any other action performed with a tool, it is possible to conclude that the aforementioned action was performed with the same tool.
Most existing automatic ballistic and/or tool mark comparison systems acquire 2D luminance images L(X,Y). Other systems acquire 3D topography images as well, that is, a relief map Z(X,Y) of an area on a ballistic piece of evidence, where Z is the local height of the surface at position (X,Y) relative to the sensor used. In most cases, the area of the ballistic piece of evidence or the tool mark piece of evidence needed for analysis purposes is larger than the field of view of the sensor used to measure the aforementioned surface characteristics. Since the area is larger than the field of view of the sensor, several 3D and 2D images are successively acquired and motion is applied to the surface to be measured between each image acquisition. The 3D images are then merged into a unique, larger image (and similarly for the 2D images).
When acquiring each individual 3D and 2D image of an object showing tool mark patterns, the surface within the field of view must be as perpendicular to the optical axis of the sensor as possible. The information relevant for surface analysis is the shape, length and depth of the mark. If the surface is not locally perpendicular to the optical axis, occlusion may occur, and the bottom of the mark, which is used to define the depth, cannot be imaged properly. Furthermore, since many of the surfaces on which tool marks are efficiently transferred are metallic in nature, and considering that the reflection of the light from a metallic surface has a strong specular contribution, most of the light reflected back to the sensor is from regions nearly perpendicular to the optical axis. For that reason, several 3D sensor technologies, including confocal ones, have a hard time finding the 3D topography of metallic surfaces which are not reasonably perpendicular to the optical axis.
When acquiring the 3D topography of an object with a perfectly cylindrical cross section, such as a pristine fired bullet, it is sufficient to rotate the object during data acquisition if the bullet is installed with its symmetry axis perfectly aligned along the rotation axis of the motor system and the starting area to be acquired is set perpendicular to the optical axis of the sensor. Simple rotation of the bullet will then ensure that the surface within the field of view of the sensor is always perpendicular to the sensor's axis. In the case of a flat surface, no rotation is necessary. The flat surface is installed with its starting area perpendicular to the sensor axis. Translational motions are then sufficient to insure that all other acquired areas also remain perpendicular to the axis.
The situation is significantly different for deformed bullets or arbitrary surfaces showing tool marks, which can display a large variety of shapes: elliptical, flat, locally concave, among others. The techniques known in the prior art cannot be applied to these arbitrary shapes as they will not ensure proper capture of the local micro topography.